Comparison of conventional versus customised Eurosil-4 Pink bolus for radiotherapy of the chest wall

Introduction In radiotherapy, the presence of air gaps near a tumour can lead to underdose to the tumour. In this study, the impact of air gaps on dose to the surface was evaluated. 3D-printing was used to construct a Eurosil-4 Pink bolus customised to the patient and its dosimetric properties were compared with that of Paraffin wax bolus. Methods Surface dose was measured for flat sheets of Eurosil-4 Pink bolus with different thicknesses. Different air gap thicknesses were inserted between the bolus and the surface, and dose was measured for each air gap using 10 cm × 10 cm fields. This was repeated with the effective field size calculated from the patient plan. Surface dose was measured for varying angles of incidence. A customised chest phantom was used to compare dose for two customised Eurosil-4 Pink boluses, and commonly used Paraffin wax bolus. Results The surface dose was found to be highest for 1.1 cm thick bolus. The decrease in surface dose for the Eurosil-4 Pink bolus was minimal for the 10 cm × 10 cm field, but higher for the effective field size and larger angles of incidence. For instance, the dose was reduced by 6.2% as a result of 1 cm air gap for the effective field size and 60 degree angle of incidence. The doses measured using Gafchromic film under the customised Eurosil-4 Pink boluses were similar to that of the Paraffin wax bolus, and higher than prescribed dose. Conclusions The impact of air gaps can be significant for small field sizes and oblique beams. A customised Eurosil-4 Pink bolus has promising physical and dosimetric properties to ensure sufficient dose to the tumour, even for treatments where larger impact of air gaps is suspected.

PrusaSlicer is a slicing software compatible with Prusa 3D-Printers. It is used to convert the 3D models from STL (stereolithography) file format into G-code. Its major benefits are custom print settings, filament settings and printer settings.
3DSlicer (Brigham and Women's Hospital, Boston, MA, USA): https://download.slicer.org/ 3D Slicer offers efficient analysis of medical images. The software contains over 100 built-in modules and more can be installed from extension manager. DICOM files can be imported from the DICOM module. The data can be viewed in 3D from anterior, posterior, superior as well as inferior orientations. Volume rendering module helps to display the 3D volume of the data. The segment editor module allows the user to perform contouring of anatomical structures. The segmented data can be converted to 3D surface models using the model maker module. Other important modules are markups, registration, quantification, diffusion, etc.
Fusion 360 (Autodesk, San Rafael, CA, USA): https://www.autodesk.com/products/fusion-360/free-trial Fusion 360 is useful in designing and engineering 3D models. It's a computer aided design (CAD) software which enables the user to build structures with accurately known dimensions and geometries.

S1.3 Eurosil-4 Pink
Eurosil-4 Pink (SynTec, Schouten Group, Netherlands) is used as a silicone mould making tool. It has two components (A and B) that need to be added together. It should not take more than 10 minutes so as to avoid the substances from solidifying before the required quantity is poured. Both the components are used in the ratio of 1:1. The mixture is then stirred till it looks uniform and left out for 24 hours after that. The mould material has high tensile strength (4.5-5 N/mm 2 ) and tissue equivalence (density is 1.04 g/cm 3 ). It also has good skin compatibility.

Figure S3: Filling the tray with Eurosil-4 Pink to construct bolus
Autodesk Fusion 360 was used to model a tray with an internal volume of 10 cm × 10 cm × 4 cm. A scale was glued at one corner to measure height. The two components of Eurosil-4 Pink were mixed well and poured in the container till the height of 1 cm. This was left to dry for 24 hours. When extracted out, a bolus of base 10 cm × 10 cm and height 1 cm was obtained. The same procedure was followed to make boluses with the same base, and heights of 0.5 cm, 1 cm, 2 cm, and 3 cm. The bolus thicknesses were measured using a vernier caliper, 10 times for each of the 4 boluses. The thicknesses calculated were 0.56 ± 0.12 cm, 0.93 ± 0.11 cm, 2.09 ± 0.21 cm and 2.81 ± 0.17 cm. For the bolus thickness experiment, the boluses can be stacked together to get a different thickness.
A CT scan of all four boluses was taken to check if they have a uniform density. The DICOM files were imported in 3D Slicer. After volume rendering, the threshold was set as the range 47.28 to 2061.95 HU. Then using the scissors tool in the same module (with settings: operation -erase outside; shape -rectangle; slice cut -unlimited) a cuboidal segment was selected from the bolus, excluding the edges and corners. The edges were not included so as to avoid beam hardening CT artefact. The segment statistic module was used to calculate the mean and standard deviation of Hounsfield units (HUs) of the segment for each bolus. We got 249.73 ± 19.90 HU for 0.5 cm bolus, 239.30 ± 12.41 HU for 1 cm bolus, 244.24 ± 16.67 HU for 2 cm bolus and 241.30 ± 14.22 HU for 3 cm bolus. This confirmed that the boluses had a uniform density. Figure S4: Process for construction of ion chamber sheet

S2.2 Eurosil-4 Pink sheet to insert ion chamber
The CC04 ionisation chamber was added in the setup with the help of one Eurosil-4 Pink sheet that had an insert for the ion chamber. The Eurosil-4 Pink sheet was preferred for inserting the ion chamber instead of solid water because it is the same material as the Eurosil-4 Pink boluses; thus more suitable for dose calculations. In order to construct this sheet, a hole of the size of ion chamber was drilled in the tray which was previously used for bolus construction. The ion chamber was fitted such that it would be close to the surface of the sheet. Eurosil-4 Pink was filled in and left to dry for 24 hours.

Figure S5: Setting up the SAD and other setup parameters
10 cm of solid water was kept on the LINAC couch for backscatter, and the Eurosil-4 Pink sheet containing the ionisation chamber was centered on it with the light field. The temperature was measured with a thermometer. The SAD was set as the distance from the source to the center of the height of the ion chamber sheet, and was equal to 100 cm. A 6 MV beam was used in all the measurements.

S2.4 Checking optimal bolus thickness
At first, the setup was irradiated without any added bolus. Afterwards, irradiation was performed for separate cases of the following thickness of added bolus: (i) 0.5 cm (ii) 1 cm (iii) 1.5 cm (iv) 2 cm (v) 2.5 cm (vi) 3 cm (vii) 3.5 cm (viii) 4 cm. 200 MUs were delivered and the field size was 10 cm × 10 cm. Three readings were taken from the electrometer for each case. The air gap was inserted between the ion chamber sheet and the bolus. A used radiochromic film was inserted on top of the air gap stand to prevent the bolus from bulging into the air gap, and the films thickness was considered in the calculations. Irradiations were performed separately for no air gap used and the different cases of air gaps (i) 0.1 cm (ii) 0.2 cm (iii) 0.3 cm (iv) 0.4 cm (v) 0.5 cm (vi) 0.8 cm (vii) 1 cm (viii) 1.5 cm. 200 MUs were delivered, and field size was 10 cm × 10 cm. Three readings were recorded from the electrometer for each case.
In the next part, the field size was set to the average field size found for the patient plan, i.e., 2.8 cm × 22.7 cm. The same setup was repeated, and dose was delivered for each air gap separately. 50 MUs were used, and three readings were recorded for each case.
In order to observe the effect of oblique beams, measurements were performed for the following angles of incidence-0°, 10°, 20°, 30°, 40°, 50°, and 60°. The field size was same as the last part of the experiment, i.e., 2.8 cm × 22.7 cm and the gantry angle was changed to a given angle. This was done for the case of no air gap and 1 cm air gap. Three readings were taken for each angle of incidence.

S2.6 Construction of customized Eurosil-4 Pink bolus
The patient CT was imported into 3D Slicer software. First, volume rendering module was used to visualize the 3D volume of the patient. A label map was created in the editor module by using the threshold effect tool, which selects the region to be included in the model. The threshold should encompass the entire chest surface. The make model effect tool was used to convert the chest label map into a 3D model, which was then saved in STL file format.

Figure S8: Model making in 3D-Slicer after volume rendering
The chest 3D model was imported into Meshmixer software. A surface region covering the tumour was selected, which was located on the chest wall. This was then extracted from the entire chest as a separate 3D layer. As this is the only layer which is required for making the bolus, the rest of the model can be cleared from the window. The 3D layer was extruded in the normal direction by a distance equal to the desired thickness of the customised Eurosil-4 Pink bolus. It was then cut to the required shape, and smoothed to remove the artefacts and CT slice irregularities. The modified file was saved separately as 'bolus shell.stl'.

Figure S9: Extracting a surface layer in Meshmixer
'bolus shell.stl' was imported in Prusa Slicer (Prusa Research, Czech Republic). The print settings were set as 0% infill, spiral vase, 1 raft layer, and PLA was chosen as the filament. A 3D-printed hollow bolus shell was obtained, which was then filled with Eurosil-4 Pink. After 24 hours the shell was cracked open to get the Eurosil-4 Pink bolus, customized to the patient chest surface. Two thicknesses of Eurosil-4 Pink bolus (0.6 cm and 1.1 cm) were made from this procedure. Figure S10: Adjusting print settings in PrusaSlicer (Such as 0% infill)